Circuit arrangement for the evaluating a binary signal defined by current threshold values

ABSTRACT

A circuit arrangement for evaluating a binary signal defined by two current thresholds, particularly the output signal of an active sensor [( 1′ )], comprises a current source [(IQ′_,IQ 11 ,IQ 12 ,IQ 13 )] that can consist of individual current sources and is connected in series to the signal source, namely the sensor [( 1′ )]. The current source is inserted between the battery terminal [(IGW)] and the sensor terminal [(A 1 )] and serves simultaneously as a current limiter in case of a short circuit between the sensor terminal [(A 1 )] and ground [(GND)].

TECHNICAL FIELD

The invention pertains to a circuits for sensing binary signals.

BACKGROUND OF THE INVENTION

Such a circuit arrangement is described in the older, not previouslypublished German Patent Application No. 195 10 055.7. Specifically thisdiscusses an active binary sensor as the source for this binary signal.This sensor generates a square-wave signal based on two differentcurrent values or current thresholds. The frequency of this square-wavesignal contains the information to be measured.

From the older, not previously published DE 44 34 34 180 A1 a circuitarrangement is known for evaluating a binary signal, thus including theoutput signal of an active binary sensor, in which a signal currentproportional to the sensor current is obtained with the aid of acurrent-mirror circuit. This signal current is tapped by way of an ohmicresistor from a constant-voltage source, whereby a binary voltage signalproportional to the sensor output voltage is generated. Thecurrent-mirror circuit thus serves for converting the current signal ofthe sensor into a binary voltage signal which can then be furtherprocessed with small effort and above all with a low power requirement.

Active sensors of the type under discussion here can be employed, forinstance as wheel sensors for determining the rotational behavior of oneof the individual wheels of a motor vehicle. The wheel rotationalbehavior is a particularly important input parameter for motor vehiclecontrol systems which are utilized, for instance, for antilockingprotection, for control of drive slippage, vehicle stability, and so on.

These control systems or some of their functions are consideredsafety-critical, because in case of a defect, the braking function ordriving stability may be compromised. Numerous monitoring measures,error displays and so on are therefore prescribed.

Particular value is placed on designing the terminal wires or terminalpins with which the controller of ABS is equipped and which serves forconnecting the wheel sensors and other components or the associatedcircuits to be short-circuit resistant. Since the loads in a motorvehicle are generally connected to one terminal of the vehicle batteryor current generator by way of the vehicle chassis, this means that thesecond individual terminals or terminal pins lead to the second terminalof the battery. This applies also to the terminal pins of the sensors.

Therefore, when connecting the sensor terminal pins to the chassis, ashort-circuit or overload protection must be in place. This has theconsequence in practice that the terminals leading to the battery or anindividual terminal of the battery (the positive pole, for instance)must be directed by way of a current-limiting circuit, which upon theoccurrence of a short circuit restricts the current to a permissiblevalue.

It is also prescribed that several mutually independent terminals of thesensors be present, so that, in case of a sensor defect or a shortcircuit, the effect of this short circuit is limited and the othersensors or at least some of the other sensors remain functional.Consequently, each of the terminals to be protected must be equippedwith a separate current limiter.

The invention overcomes the problems associated with reducing therequired effort for short-circuit-proof layout of the terminal pins of acircuit arrangement of the type mentioned initially, without having toaccept disadvantages of other type.

This problem can be solved by the present invention. The particularityof the circuit arrangement according to the invention is thus that in acircuit system with a ground connection, as found in motor vehicles, thecurrent source used for evaluating the sensor signal is inserted betweenthe battery or individual terminal and the sensor terminal and serves asa current limiter and thus as overload protection in case of a shortcircuit or shunt between the sensor pin and ground.

Due to the mode of connecting the sensor or current source according tothe invention, the current limiter circuit, which previously had to beinserted between the terminal pin and the battery terminal, becomessuperfluous. Beyond that, an individual protection of each sensorterminal against short circuits is achieved, because each sensor or eachsensor evaluation circuit is equipped in any case with a separatecurrent source for evaluating the sensor signal.

Using the current limiter of the present invention results in asignificant savings. In contrast to conventional current limiters, thecircuit arrangement according to the invention can be implemented veryeasily by an integrated circuit. This integrated circuit can then beprotected in a very simple manner by a thermal fuse, also constructedand integrated on a semiconductor basis, against overload by excessivelyhigh loss power caused, for instance, by a continuing short circuit.

Several particularly advantageous embodiments of the circuit arrangementaccording to the invention are described in the subordinate claims.

Additional characteristics, advantages and application possibilities ofthe invention can be obtained from the following description ofembodiments on the basis of the attached figures.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematically simplified, a circuit arrangement forevaluating the output signal of an active sensor.

FIG. 2 shows a circuit arrangement presented in the same manner toillustrate error detection.

FIG. 3 shows a part of an integrated circuit for implementing thecircuit of FIG. 2.

FIG. 4 shows an embodiment of a circuit arrangement according to theinvention presented in the same manner as FIG. 2.

FIG. 5 shows a current source of circuit arrangement according to FIG.4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The basis for the embodiment according to FIG. 1 is the use of an activerpm sensor 1 in a motor vehicle. Such a sensor 1 can be a component of amotor vehicle control system. A signal whose frequency is proportionalto the rotational velocity of the respective wheel can be obtained withthe aid of such sensors or wheel sensors.

In the illustrated example the measurement element is an active sensor 1whose output is preferably bound by two current threshold values, namelya lower current level of 7 mA and an upper current level of 14 mA. Thelower level can, however, be set between 5 and 8 mA and the higher levelcan be set from 11 to 18 mA. The lower current is necessary in order tomaintain the orderly functioning of the active sensor 1. A terminal IGN,through which the connection to the positive terminal of the vehiclebattery can be produced upon switching on the ignition, serves as acurrent source here. The ground connection leads to the negativeterminal of the battery.

The wheel sensor 1 is shown here as a current source which is composedof two individual current sources. One of these individual currentsources supplies the low current I_(L)=I_(s1), which is supplemented toform the high current I_(H)=I_(s1)+I_(s2) in the high phase of thesignal by connecting the second individual current source or anadditional current source I_(s2).

A second current source IQ designed for a nominal current I₀ isconnected in series with the active sensor 1. In more precise terms,this current source is a current sink as will become evident from theexplanations below.

The nominal current I₀ lies above the lower current threshold value ofthe sensor 1, i.e., the low current I_(L). It is practical for thenominal current I₀ from the current source IQ to be chosen to correspondto an intermediate value between the two current threshold values I_(L)and I_(H). An amplifier stage, here. the base-emitter junction of atransistor T, is connected in parallel with the current source IQ. Thevoltage drop across the current source IQ is simultaneously the inputvoltage U_(E) of the amplifier stage T. The circuit according to FIG. 1functions as follows.

So long as the current from the sensor 1 is below the nominal current,or the applied current from the current source IQ, which applies for thelow phase of the sensor 1, the potential U_(E) at the input of theamplifier circuit T is reduced by the current source IQ almost to groundpotential GND. The transistor T is not conducting. The output signal, orthe output potential U_(A) of the amplifier circuit, that is, thepotential at the collector of the transistor T is high; the outputsignal U_(A) takes on the full value of the supply voltage V_(cc5).

However, as soon as the sensor current I rises above the nominal currentI₀ of the current source IQ, the transistor T conducts. This is the casein the high phase, when the sensor 1 is supplying the high current I₀.The current source IQ is only capable of sinking its nominal current I₀.Any current above that leads to an increase in the potential U_(E),conduction of the transistor T and a low value of the output signalU_(A). In this phase the input potential U_(E) is limited by a Zenerdiode Z in parallel with the current source IQ. Having the current flowthrough the Zener diode Z also ensures that a current IH which issufficient for the operation of the sensor 1 can flow.

The circuit according to FIG. 1 can be expanded simply and with littleexpense to a circuit that is capable of recognizing and displayingsensor errors. These sensor errors also include a short circuit of theconnection line to ground (GND) or battery (IGN), an interruption of theline and shunts. The mode of operation of such a circuit with errorrecognition is illustrated in FIG. 2. This expansion is achieved by adivision of the second current source (IQ in FIG. 1) into several—here,three—individual current sources IQ₁, IQ₂, IQ₃. The potential acrossthese current sources is determined in each case with aparallel-connected amplifier stage, symbolized by the transistors T₁,T₂, T₃, Diodes D1 and D2 serve to decouple the individual currentsources.

The individual current sources IQ₁, IQ₂ and IQ₃ are connected togetherand to the active sensor 1 such that the first individual current sourceIQ₁ connected directly to the sensor 1 signals a power interruption or asensor current lying below a minimum level. In the present example, IQ₁is designed for a minimum current of I₁=3 mA, so that the associatedamplifier stage T₁ is triggered only if the signal current or sensorcurrent I exceeds this level. A “high” at the output X₁ of theassociated amplifier circuit T₁ consequently indicates a lineinterruption or a sensor current I which is too low for another reason.

The next, via the diode D1, individual current source IQ₂, which isdesigned here for a near value nominal current I₂=7 mA but can be around10 mA, becomes conductive as soon as the sensor current exceeds theminimum value I₁. At the output of the amplifier circuit T₂, which is inparallel with the individual current source IQ₂, a high signal ispresent until the sensor current reaches or exceeds the sum of thenominal currents I₁+I₂ of the two current sources IQ₁ and IQ₂. Only thendoes the signal at output X₂ of stage T₂ change from “high” to “low.”Since the sum (I₁+I₂) of the nominal currents of the above-describedindividual current sources IQ₁ and IQ₂ exceeds the lower thresholdcurrent value I₂ of the wheel sensor 1, but the nominal current of theindividual current source IQ₂ is not reached in the low phase of thesensor, the evaluative voltage signal, which represents the result ofthe current-voltage transformation and corresponds to the output signalU_(A) of FIG. 1, is available at the output of the amplifier stage T₂ inregular operation of the wheel sensor 1, that is, in case of constantalternation of the sensor signal current between the lower currentthreshold value (I₁) and the upper one (I₂).

The third individual current source IQ₃ according to FIG. 2 serves tosignal an excessively high sensor current due to a fault or anexcessively high input current into the evaluation circuit. Anexcessively high current can be produced by a shunt or even a shortcircuit to the supply terminal IGN. The nominal current of the thirdindividual current source IQ₃ determines the maximum value, which can bearound 18 mA value. If the sum I₁+I₂+I₃ of the nominal currents of theindividual current sources IQ₁, IQ₂, IQ₃ is exceeded, this results in atriggering of the amplifier stage T₃ and therefore in a change of thesignal at output X₃ of this amplifier stage from “high” to “low.”

FIG. 3 shows an example of the implementation of the circuit accordingto FIG. 2. All the illustrated components are parts of an integratedcircuit. The individual current sources IQ₁′, IQ₂′ and IQ₃′ areimplemented here with current-mirror circuits. The nominal current orapplied current is specified in familiar manner by appropriate selectionof the ohmic resistors R₁,R₂,R₃ and specifying the supply voltageU_(REF). From the potential at outputs X₁′ and X₃′ of the amplifiercircuits T₁′ and T₃′ it can again be recognized in the manner describedon the basis of FIG. 2 whether a sensor fault is present. The convertedsensor signal is available for further processing at output X₂′ of theamplifier circuit T₂′.

The voltage U_(REF) required for setting the nominal currents isstabilized in each case, while an unstabilized or only roughlystabilized voltage would suffice in many application cases for thesupply voltage V_(cc5).

It can be recognized from the foregoing description that the inventioncan be implemented particularly well in the form of integrated circuits.Only a few components are required for signal evaluation and faultrecognition. The power consumption is low. An essential advantage isthat no high requirements need be placed on the precision of thecomponents and the setting of the current thresholds. This has afavorable effect on the production costs for such a circuit arrangement.Moreover, high reliability in operation can be expected for the samereasons. Since the nominal currents of the individual current sourcesand thus the circuit thresholds for fault recognition can be changedeasily and with little expense, by adjusting the reference voltage, forinstance, an adaptation to sensors of different types is easilypossible.

Unlike the circuit arrangement according to FIG. 2, a current sourceconsisting of the individual current sources IQ₁₁,IQ₁₂ and IQ₁₃ isinserted in the embodiment according to FIG. 4 between the batteryterminal IGN—generally the positive pole—and a terminal or terminal pinA1 serving to connect the associated active sensor 1′. A dotted-dashedline symbolizes the separation of the electronic circuit componentshoused in a regulator and shown at the right from terminal pin A1 shownat the left, to which the active sensor 1′ is connected via a signalline. The second sensor terminal is connected to ground GND. The currentsource IQ′ and the active sensor 1′ are connected in series like thecircuits explained in the aforementioned FIGS. 1-3.

In the normal case, the individual current source IQ₁₂ or the associatedamplifier circuit T₁₂ makes the sensor signal available at its outputX₁₂, in the same manner as the current source IQ₂ with the amplifiercircuit T₂ of the circuit according to FIG. 2. The current source IQ₁₁and the associated amplifier circuit T₁₁ signal a line interruption oran open terminal pin A1, while the individual current source IQ₁₃ bringsabout a change of the signal at the output X₁₃ of the amplifier circuitT₁₃ in case of short circuit or a shunt between the terminal A1 andground GND.

The maximum possible short circuit current that appears in case of ashort circuit of the terminal A1 to ground is given by the sum of theapplied currents of the three individual current sources IQ₁₁,IQ₁₂,IQ₁₃,that is, I₁₁+I₁₂+I₁₃. The presence of a short circuit is indicated bythe output signal X₁₃. The illustrated circuit, which is preferablyproduced by integration technology, is designed for at least ashort-term load with this short-circuit current. It is possible for atemperature-dependent semiconductor element that shuts off the currentsupply in conventional manner to be built into the integrated circuit tocounteract the thermal load when the short circuit is maintained.

FIG. 5 shows details of the individual current source IQ₁₁; the otherindividual current sources have the same structure. Here again, asalready described on the basis of FIG. 3, a current-mirror circuit isemployed. The magnitude of the ohmic resistor R₁₁ and the level of thereference voltage U_(REF) determine the current that flows through thetransistor T₁₁₃ connected as a diode. The current mirroring through thetransistor T₁₁₄, which in turn determines the current through transistorT₁₁₂, and, through another current-mirroring, the current through thetransistor T₁₁₁ has the consequence that the current set by the resistorR₁₁ and the reference voltage U_(REF) becomes the nominal current orapplied current of the current source IQ₁₁. The nominal current of thecurrent source is labeled I₁₁ here. The other two individual currentsources shown in FIG. 4 are constructed in exactly the same way.

In an embodiment an active sensor 1′ was employed with the currentthreshold values 7 mA/14 mA. In this case, the nominal currents of thethree individual current sources IQ₁₁,IQ₁₂,IQ₁₃ were each set at 5 mA.The 7 mA sensor current here is composed of the nominal current I₁₁=5 mAof the individual current source IQ₁₁ and a current of 2 mA fed via thediode D₁₁, which the individual current source IQ₁₂ supplies. If theupper current threshold of 14 mA is reached, this current is composed ofthe nominal currents of the sources IQ₁₁ and IQ₁₂ and a differentialcurrent of 4 mA which the source IQ₁₃ supplies.

In case of a short circuit of sensor 1′, the current can rise only to 15mA.

What is claimed is:
 1. A circuit for evaluating a sensor signal from asensor, the circuit setting a lower and an upper level for the sensorsignal, the circuit comprising: a signal source that acts as the sensor,the signal source including a first current source, the first currentsource setting an upper current threshold and a lower current threshold,the second current source being coupled to said first current source andgenerating a nominal current, the nominal current falling in between theupper and lower current threshold levels, wherein a voltage across thesecond current source indicates a signal state of the signal source, thesecond current source including at least two individual current sourcescoupled together; and at least two amplifier circuits connected to saidat least two individual current sources in said second current source,each individual current source having an associated amplifier circuit,wherein output signals in the amplifier circuits indicate whether thenominal current from the first current source from the sensor fallsbelow the lower current threshold level or extends above the nominalcurrent, and wherein said first and second current sources and saidamplifier circuits operate together as a current limiter and an overloaddetector in case of a short circuit or shunt to ground.
 2. The circuitaccording to claim 1, wherein the second current source is composed offirst, second and third individual current sources that are connected inparallel with decoupling diodes inserted between the individual currentsources, wherein the first individual current source triggers when thesensor signal falls below a minimum current value, the second individualcurrent source triggers when the sensor signal falls between a meancurrent value and the minimum current value and the third individualcurrent source triggers when the sensor signal falls between a maximumcurrent value and the mean current value.
 3. The circuit according toclaim 2 wherein the lower current threshold and the upper currentthreshold are set by two individual current sources in said firstcurrent source.
 4. The circuit according to claim 1, wherein the secondcurrent source includes a plurality of individual current sources, eachindividual current source being constructed with a currentmirror circuitand wherein nominal currents of the individual current sources arederived from a common reference voltage source via ohmic resistors andadditional nominal current circuits.
 5. The circuit according to claim3, wherein the lower threshold is roughly 3 mA, the mean current valueis roughly 10 mA and the upper threshold is roughly 17 mA.